Dielectric isolators

ABSTRACT

The present application is directed to dielectric isolators for use in aircraft fuel systems to control lightning induced current and allow dissipation of electrostatic charge. The dielectric isolators are configured to have a high enough impedance to limit lightning currents to low levels, but low enough impedance to allow electrostatic charge to dissipate without allowing buildup. Although the dielectric isolators may develop a potential difference across the dielectric length due to the effects of lightning currents and its inherent impedance, they are configured to withstand these induced voltages without dielectric breakdown or performance degradation. In one embodiment, the dielectric isolator includes a tube constructed of a composition including a thermoplastic organic polymer (e.g., PEEK) and carbon nanotubes, and a pair of couplings attached to opposing ends of the tube. The dielectric isolator is capable of exhibiting an electrical resistance from about 10 5 Ω to 10 8 Ω at an applied potential of greater than 500 volts DC when measured from one coupling to the other coupling.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No. 12/166,698 filed on Jul. 2, 2008, the disclosure of which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present application pertains to dielectric isolators for use in fluid conveyance applications and, more particularly, to dielectric isolators that serve to control lightning induced current and allow dissipation of electrostatic charge in aircraft fuel systems.

BACKGROUND

Dielectric fittings or isolators are known in the art and have found use in many applications, ranging from natural gas pipelines, where they isolate monitoring instruments from the effects of electrical current and interrupt cathodic current flow while permitting fluid flow, to providing a conduit for transferring liquid through selected locations as required or aircraft bulkhead structures. In the latter usage, the dielectric fitting contains integral fitting connections on both sides of the aircraft bulkhead that permit connections of tubes, hoses, or other fluid-carrying components. Such a dielectric fitting also provides a high electrical resistance path that limits electrical current flow between the two fitting connections, but allows for the gradual dissipation of electrostatic charge.

Thus, the primary functions of a dielectric fitting, also referred to as a static dissipative isolator, is to dissipate the electrical energy from static charges caused, in part, by fluid movements and to limit electrical current flow caused by the indirect effects of lightning, at such an occurrence. These fittings have the equally important secondary function of providing a safe fluid passage for the fluid passing through the fuel tank or other areas of the aircraft.

SUMMARY

In one embodiment, a fluid carrying element for use in a dielectric isolator is disclosed. The fluid carrying element includes a tube constructed of a composition including PEEK and carbon nanotubes, wherein the tube composition exhibits a bulk volume resistivity from about 10³ Ω-cm to 10¹⁰ Ω-cm at an applied potential of 5,000 volts DC.

In another embodiment, a dielectric isolator is disclosed. The dielectric isolator includes a tube constructed of a composition including a thermoplastic organic polymer and carbon nanotubes, and a pair of couplings attached to opposing ends of the tube. In this embodiment, the dielectric isolator exhibits an electrical resistance from about 10⁵Ω to 10⁸Ω at an applied potential of greater than 500 volts DC when measured from one coupling to the other coupling.

In another embodiment, an inline-type dielectric isolator for use in an aircraft fuel system is disclosed. The dielectric isolator includes a tube constructed of a composition including PEEK, carbon nanotubes, and carbon fibers, and a pair of ferrules threadingly attached to opposing ends of the tube. In this embodiment, the dielectric isolator exhibits an electrical resistance from about 10⁵Ω to 10⁸Ω at an applied potential of greater than 500 volts DC when measured from one ferrule to the other ferrule.

In another embodiment, a bulkhead-type dielectric isolator for use in an aircraft fuel system is disclosed. The dielectric isolator includes a tube constructed of a composition including PEEK, carbon nanotubes, carbon fibers, and chopped glass fiber. The dielectric isolator further includes a pair of ferrules threadingly attached to opposing ends of the tube and a flange threadingly attached to the tube between the ferrules. In this embodiment, the dielectric isolator exhibits an electrical resistance from about 10⁵Ω to 10⁸Ω at an applied potential of greater than 500 volts DC when measured from one ferrule to the other ferrule.

BRIEF DESCRIPTION OF THE DRAWINGS

It will be appreciated that the illustrated boundaries of elements in the drawings represent only one example of the boundaries. One of ordinary skill in the art will appreciate that a single element may be designed as multiple elements or that multiple elements may be designed as a single element. An element shown as an internal feature may be implemented as an external feature and vice versa.

Further, in the accompanying drawings and description that follow, like parts are indicated throughout the drawings and description with the same reference numerals, respectively. The figures may not be drawn to scale and the proportions of certain parts have been exaggerated for convenience of illustration.

FIGS. 1A and 1B illustrate a top plan and partial cross-sectional views, respectively, of one embodiment of an inline-type dielectric isolator 100.

FIG. 2 illustrates a perspective view of one embodiment of a bulkhead-type dielectric isolator 200.

FIG. 3 illustrates a perspective view of the bulkhead-type dielectric isolator 200 attached to a bulkhead 300.

FIG. 4 illustrates a graph of electrical resistance (y-axis) vs. tube diameter size (x-axis) for pre-lightning strike testing of an exemplary tube composition.

FIG. 5 illustrates a graph of electrical resistance (y-axis) vs. tube diameter size (x-axis) for post-lightning strike testing of the same exemplary tube composition.

DETAILED DESCRIPTION

The present application is directed to dielectric isolators for use in aircraft fuel systems to control lightning induced current and allow dissipation of electrostatic charge. The dielectric isolators are configured to have a high enough impedance to limit lightning currents to low levels, but low enough impedance to allow electrostatic charge to dissipate without allowing buildup. Although the dielectric isolators may develop a potential difference across the dielectric length due to the effects of lightning currents and its inherent impedance, they are configured to withstand these induced voltages without dielectric breakdown or performance degradation.

FIGS. 1A and 1B illustrate top plan and partial cross-sectional views, respective, of one embodiment of an inline-type dielectric isolator 100. The dielectric isolator 100 includes a tube 110 for conveying a fluid, such as hydrocarbon aviation fuel or hydraulic oil. In basic dimensions, the tube 110 extends axially to a predetermined length L along a central longitudinal axis A and has an inner diameter D_(i) and outer diameter D_(o). As shown in the illustrated embodiment, the tube 110 is cylindrical in shape having a circular cross-section with two opposing ends. It will be appreciated, however, that the tube 110 may be constructed in various other configurations including other cross-sectional shapes and multiple ends.

In one embodiment, the length L of the tube 110 can be from about 3 inches to 5 inches. In another embodiment, the length L of the tube 110 can be from about 2 inches to 9 inches. It will be appreciated, however, that the length can be above or below these ranges depending on the manufacturing technologies available to construct the tubes and depending on system and/or component requirements.

In one embodiment, the inner diameter D_(i) of the tube 110 can be from about 2 inches to 3 inches. In another embodiment, the inner diameter D_(i) can be from about 1.5 inches to 4 inches. It will be appreciated, however, that the inner diameter can be above or below these ranges depending on the manufacturing technologies available to construct the tubes depending on system and/or component requirements.

With continued reference to FIGS. 1A and 1B, the dielectric isolator 100 includes a pair of couplings 120 a,b attached to opposing ends of the tube 110 that are each configured to be connected to a corresponding coupling (not shown) of a separate component (not shown). It will be appreciated that any type of coupling may be attached to the end of the tube 110 including, without limitation, threaded couplings, quick connect/disconnect couplings, or other types of couplings. Furthermore, the couplings 120 a,b are not limited to a specific size and can cover a wide range of both conventional and unconventional sizes.

The couplings 120 a,b may be constructed of a metallic material, such as aluminum or stainless steel. In one specific embodiment, the couplings 120 a,b are aluminum ferrules that conform to SAE AS5836-1-XX (flex) and SAE AS1656-1-XX (rigid) and are finished with a chemical conversion coat per MIL-C-5541. It will be appreciated that the coupling material is not necessarily limited thereto, provided that the dielectric isolator 100 satisfies the mechanical and electrical requirements of a given installation.

In the illustrated embodiment, the couplings 120 a,b are threadingly attached to the ends of the tube 110 via stub ACME threads. Alternatively, the couplings 120 a,b can be adhesively bonded to the ends of the tube 110. Although many different types of adhesives may be used, one suitable adhesive that can be used to bond the couplings 120 a,b to the ends of the tube 110 is Araldite AV119 distributed by Huntsman Advanced Materials. It will be appreciated that the couplings 120 a,b can be attached to the ends of the tube 110 utilizing a variety of other attachment means. For example, the couplings 120 a,b can be riveted or swaged onto the ends of the tube 110 for attachment thereto. In all cases, a protective sealant (e.g., MIL-S-81733 or MIL-S-4383) may be applied to the faying surfaces between the couplings 120 a,b and the tube 110 to seal the adhesive bond (if present) from outside elements and to protect the metallic couplings 120 a,b from galvanic effects (depending on the material from which the tube 110 is constructed).

From a mechanical perspective, the tube 110 in the dielectric isolator 100 serves as a fluid carrying element to permit fluid to flow therethrough. However, from an electrical perspective, the tube 110 in the dielectric isolator 100 also serves as a resistor having a high and robust electrical impedance to limit indirect currents associated with lightning events to low levels across the dielectric isolator 100, but low enough surface impedance to allow electrostatic charge to dissipate without allowing buildup. In other words, the tube 110 allows enough surface current i to flow therethrough to prevent electrostatic charge build up due to fluid flow through the fuel line, yet limits the flow of current i associated with voltage v therethrough during a lightning strike to current levels that are relatively low and safe for the aircraft fuel system.

Because lightning isolation requires electrical robustness, the tube 110 must exhibit high resistance even after repeated exposure to high voltages. Some materials experience high voltage conditioning when exposed to high voltages such as those encountered in lighting isolation applications. High voltage conditioning is a phenomena that substantially decreases the effective resistance of a material after exposure to high voltages. Collapse in the effective resistance of the material increases the conductivity of the material, causing the material to substantially lose its isolator properties.

Accordingly, the tube 110 is constructed of an appropriate material that enables the tube 110 to be conductive enough to dissipate electrostatic charge build up associated with fuel flow through and over the tube 110, while at the same time enabling the tube 110 to be resistive enough to prevent the flow of electrical current therethrough associated with lightning events. Additionally, the tube composition needs to be effectively immune to high voltage conditioning for the reasons discussed above. In other words, the tube material must maintain acceptable electrical performance after being subjected to repeated lightning strike events.

To meet these electrical requirements, the tube 110 may be constructed of a composition that includes a thermoplastic organic polymer, carbon nanotubes, and, optionally, other conductive carbon materials. In order to accommodate high voltage events, such tube compositions exhibit a bulk volume resistivity of about 10³ Ω-cm to 10¹⁰ Ω-cm at an applied potential of 5,000 volts DC. Other voltages such as 1,000 volts DC or 10,000 volts DC may also be used, and the corresponding resistivity determined, however the resistivity should be approximately in the dissipating to insulating range, as described below.

Thermoplastic compositions that are subjected to high voltages tend to suffer degradation in their resistivity properties over time, in as little as one cycle when subjected to high voltage, or even during the ramp up to a high voltage. Thus, the tube compositions d herein are prepared to withstand high voltage events and maintain their resistivity properties. The phrase “bulk volume resistivity degradation,” is used to refer to one measure of the overall degradation in resistivity due to high voltage events.

The bulk volume resistivity degradation of a composition is determined at a specified voltage (e.g. 100 volts DC, 500 volts DC, 1,000 volts DC, 5,000 volts DC, or other predetermined voltage), after a specified number of cycles. Such resistivity changes may be referred to as a five-strike bulk volume resistivity degradation, where five is the specified number of cycles. For example, a five-strike bulk volume resistivity degradation at 5,000 volts DC would be determined by subjecting the material to cycling between zero and 5,000 volts DC for five cycles, and measuring the resistivity at 5,000 volts DC during the first and fifth cycles. Desirably, the present compositions exhibits a five-strike bulk volume resistivity degradation of no more than a 20× factor after being cycled between zero and the specified voltage. In some embodiments, the compositions exhibit a five-strike bulk volume resistivity degradation of no more than a 15×, 10×, or even a 5× factor after being cycled between zero and the specified voltage, e.g., 5,000 volts DC. For example, if the maximum degradation is a 20× factor and the initial determination at the specified voltage is a bulk volume resistivity of 6×10⁸ Ω-cm, the bulk volume resistivity would be no less than 3×10⁷ Ω-cm at the fifth cycle. In other embodiments, the present compositions may exhibit a five-strike bulk volume resistivity degradation of no more than 50% after being cycled between zero and the specified voltage, e.g., after being cycled five (5) times between zero and 5,000 volts DC. In some embodiments, the present compositions exhibit a five-strike bulk volume resistivity degradation of no more than a 25%, 10%, 5%, or even as low as a 2% after being cycled between zero and the specified voltage, e.g., 5,000 volts DC. For example, when the composition exhibits a maximum degradation in its five-strike bulk volume resistivity degradation of no more than a factor of 50%, if the initial determination at the specified voltage is a bulk volume resistivity of 6×10⁸ Ω-cm, the bulk volume resistivity would be no less than 3×10⁸ Ω-cm at the fifth cycle; correspondingly for a maximum degradation of 25%, the bulk volume resistivity would be no less than 4.5×10⁸ Ω-cm at the fifth cycle.

As used herein, volume resistivity is the resistance (ohm-cm) to leakage current through the body of an insulating material. As used herein, surface resistivity (ohms per square) is defined as the resistance to leakage current along the surface of an insulating material. As the surface/volume resistivity increases, the leakage current and the conductivity of the material decrease.

As a reference point for volume resistivity, conductive materials such as metals are those materials having a volume resistivity of less than 1×10⁻⁴ Ω-cm. In conductive materials, charges go to ground or to another conductive object that the material is in close proximity to or contacts. Electrostatic shielding materials have a volume resistivity of less than 1×10³ Ω-cm, but greater than or equal to 1×10⁻⁴ Ω-cm. Dissipative materials have a volume resistivity of from about 1×10³ Ω-cm to 1×10¹⁰ Ω-cm. For these materials, the charges flow to ground more slowly and in a somewhat more controlled manner than with conductive materials. For ESD-sensitive (electrostatic discharge) devices, these materials provide Faraday cage protection from energy transfer. Insulative materials are defined as those having a volume resistivity of at least 1×10¹⁰ Ω-cm. Insulative materials prevent or limit the flow of electrons across their surface or through their volume, have a high electrical resistance, and are difficult to ground. Static charges remain in place on insulative materials for an extended period of time. Table 1 sets forth volume and surface resistivities for the above materials.

TABLE 1 Volume and Surface Resistivities for Material Types. Surface Resistivity Volume Resistivity Material Type (Ω/sq) (Ω-cm) Conductive <1 × 10⁻⁴ <1 × 10⁻⁴ Electrostatic ≧1 × 10⁻⁴, <1 × 10⁴ ≧<1 × 10⁻⁴, <1 × 10³ shielding Dissipative ≧1 × 10⁶, <1 × 10¹¹ ≧1 × 10³, <1 × 10¹⁰ Insulative ≧1 × 10¹¹ ≧=1 × 10¹⁰

As discussed above with respect to strike resistivity degradation, the tube compositions disclosed herein do not exhibit electrical “burn-in,” or at least exhibit a substantially reduced “burn-in” as compared to other materials. “Burn in” refers to a phenomenon where materials, once exposed to a high voltage, do not return to their initial electrical resistance, but instead exhibit a reduced resistance. Compounds formulated without carbon nanotubes tend to display such a “burn in” phenomenon. However, surprisingly, the tube compositions disclosed herein containing carbon nanotubes, eliminate, or at least substantially reduce, such “burn in.” Without being bound by theory, it is believed that compositions without carbon nanotubes, or high carbon nanotube loadings, are susceptible to material degradation and ionization when exposed to high voltages. Whereas, in the tube compositions disclosed herein, the carbon nanotubes provide an electrical conduit through the polymer matrix, when the compositions are exposed to a high voltage, such that ionization and degradation of the polymer matrix are avoided or at least substantially reduced. In other words, at high applied potentials, the compositions act as electrostatic dissipaters that allow a controlled release of a static charge build-up, thereby preventing, or at least substantially reducing, arcing or rapid discharge. Eliminating or substantially reducing the potential for generation of a spark from the compositions or the materials to which they are in contact, helps to reduce the risks for harm to the surrounding environment and people in close proximity to such a charge.

As discussed above, the tube compositions disclosed herein are insulative at low applied potentials, but is mildly conductive, i.e., the composition become dissipative, at high applied potentials. Thus, in one embodiment, by utilizing ASTM D-257, the tube composition is determined to be an insulator. As such, at an applied potential of 100 volts DC, the tube composition exhibits a volume resistivity of greater than or equal to 10⁹ Ω-cm and, more typically, at least about 10¹⁰ Ω-cm. However, in other embodiments, the tube compositions, at potentials of 1,000 volts DC, or greater, exhibit a volume resistivity of about 10³ Ω-cm to 10¹⁰ Ω-cm. The use of the standard protocol and specimen size with voltages higher than 100 volts DC, for example, over 1,000 volts DC, are referred to as a “modified ASTM D-257 protocol.” In one embodiment, a modified ASTM D-257 protocol results in the measurement of the bulk volume resistivity of the tube composition at 5,000 volts DC. The tube compositions disclosed herein desirably exhibit a bulk volume resistivity of about 10⁶ Ω-cm to 10¹⁰ Ω-cm at 5,000 volts DC.

ASTM D-257 is a standardized method for determining surface and volume resistivity of a material. Briefly, according to this method, a standard size specimen is placed between two electrodes, and a voltage is then applied for sixty seconds and the resistance is measured. Surface or volume resistivity is calculated, and apparent value is given for the sixty second electrification time. The method is well known and the full standard is available from ASTM International, West Conshohocken, Pa. In the determinations referred to in the present application, standard specimens having a standard ASTM D3801 flame bar shape and dimensions of 6″×0.5″×0.125″ were formed from the present tube composition by injection molding.

In one embodiment, the thermoplastic organic polymer used in the tube composition is polyetheretherketone (PEEK). PEEK and other polyetherketones or polyetherketoneketones are described in patents such as EP 0 001 879, U.S. Pat. No. 6,909,015, and U.S. Pat. No. 6,274,770. The thermoplastic polymer may be present in the tube composition from about 50 wt % to 98 wt % in some embodiments, from about 55 wt % to 95 wt % in other embodiments, or from about 60 wt % to 90 wt % in yet other embodiments.

Without being bound to a single example, one commercially available PEEK is Victrex 150 P available from Victrex USA, West Conshohocken, Pa. This PEEK material is commercially available as a powder that is a low viscosity grade of PEEK for use in extrusion compounding. PEEK is commercially available in a number of other grades and forms ranging from low medium and standard viscosity grades to easy flow and general purpose grades. The various grades of PEEK are known and all are flame resistant, and insulative. PEEK also exhibits wear resistance, low friction, and good chemical resistance, particularly to various fuels and other hydrocarbons. Aromatic PEEK, such as 4-(4-(4-oxyphenoxy)-phenoxy)-benzoyl polyetheretherketone, may be processed via extrusion or injection molding at polymer temperatures of about 360° C. to 400° C.

In other embodiments, the thermoplastic organic polymer used in the tube composition can include other polymers typically used in injection molding applications. For example, such polymers include, but are not limited to, polyacetal, polyacrylic, polyalkylene terephthalate, polyalkylene terephthalate glycol, polycarbonate, polystyrene, polyester, polyamide, polyamideimide, polyarylate, polyarylsulfone, polyethersulfone, polyphenylene sulfide, polyvinyl chloride, polysulfone, polyimide, polyetherimide, polytetrafluoroethylene, polyetherketone, polyetheretherketone, polyetherketoneketone, polybenzoxazole, polyoxadiazole, polybenzothiazinophenothiazine, polybenzothiazole, polypyrazinoquinoxaline, polypyromellitimide, polyquinoxaline, polybenzimidazole, polyoxindole, polyoxoisoindoline, polydioxoisoindoline, polytriazine, polypyridazine, polypiperazine, polypyridine, polypiperidine, polytriazole, polypyrazole, polypyrrolidine, polycarborane, polyoxabicyclononane, polydibenzofuran, polyphthalide, polyacetal, polyanhydride, polyvinyl ether, polyvinyl thioether, polyvinyl alcohol, polyvinyl ketone, polyvinyl halide, polyvinyl nitrile, polyvinyl ester, polysulfonate, polysulfide, polythioester, polysulfone, polysulfonamide, polyurea, polyphosphazene, polysilazane, polybutylene terephthalate, polyethyleneterephthalate glycol, or a blend of any two or more.

Carbon nanotubes, suitable for use in the tube composition, include both multi-wall and single wall nanotubes. Carbon nanotubes may be prepared using a variety of techniques, including, but not limited to, arc discharge, laser ablation, high pressure carbon monoxide (HiPCO), chemical vapor deposition (CVD), and catalytic chemical vapor deposition (CCVD). Such techniques may utilize vacuum techniques or processing gases. Carbon nanotubes can be of a wide variety of sizes. Single-walled carbon nanotubes have a shape that is reminiscent of a tube made by rolling a sheet of paper one time such that opposites sides of the sheet meet and abut one another. Whereas multi-walled tubes have a shape that is reminiscent of a scrolled sheet of paper, or chicken wire, that is rolled multiple times. The inner core region of the carbon nanotube may be hollow, or may comprise carbon atoms which are less ordered than the ordered carbon atoms of the outer region. As used herein, the phrase “ordered carbon atoms,” refers to graphitic domains having their c-axes substantially perpendicular to the cylindrical axis of the carbon nanotube. The individual graphitic carbon layers are concentrically arranged around the long axis of the fiber like the growth rings of a tree, or like a scroll of hexagonal chicken wire. There is usually a hollow core a few nanometers in diameter, which may be partially or wholly filled with less organized carbon. Each carbon layer around the core may extend as much as several hundred nanometers. The spacing between adjacent layers may be determined by high resolution electron microscopy, and in some embodiments is only slightly greater than the spacing observed in single crystal graphite, i.e., from about 0.339 to 0.348 nanometers.

The term “carbon nanotubes” refers to an essentially cylindrical, discrete carbon tube, on the nanometer scale. Single-walled carbon nanotubes typically have a substantially constant diameter from about 2 nm to 100 nm. For example, from about 3 nm to 75 nm, from about 4 nm to 50 nm, from about 4 nm to 25 nm, or from about 5 nm to 15 nm, according to various embodiments. In one embodiment, the carbon nanotubes have a diameter from about 3.5 nm to 70 nm. The length of the carbon nanotubes may also vary and may cover a range from hundreds of nanometers to the micron range. Typically, the length of a carbon nanotube is greater than about 100 times the diameter. For example, carbon nanotubes may have a length from about 100 nm to 10 μm, from about 500 nm to 5 μm, from about 750 nm to 5 μm, from about 1 μm to 5 μm, or from about 1 μm to 2 μm, according to various embodiments. An outer region of single-walled or multi-walled nanotubes is made of essentially continuous layers of ordered carbon atoms and a distinct inner core region. In multi-walled tubes, each of the layers and core is disposed substantially concentrically about the cylindrical axis of the carbon nanotube. Preferably, the entire carbon nanotube is substantially free of thermal carbon overcoat.

As used herein, the term “cylindrical” is used herein in the broad geometrical sense, i.e., the surface traced by a straight line moving parallel to a fixed straight line and intersecting a curve. A circle or an ellipse are but two of the many possible curves of the cylinder.

The purity of the carbon nanotubes may also play a role in the effectiveness of the carbon nanotubes in the tube composition. The carbon nanotubes may have a purity of greater than 50%, in some embodiments, of greater than 60%, in some embodiments, of greater than 70%, in some embodiments, of greater than 80%, in some embodiments, or of greater than 90%, in still other embodiments. Typically, the carbon nanotubes, due to manufacturing methods, contain metal oxide impurities. While the metal oxide impurities may or may not affect tube composition performance in some embodiments, the metal oxide content of the carbon nanotubes is less than 50%. In other embodiments, the metal oxide content of the carbon nanotubes is less than 60%, less than 40%, less than 30%, or less than 20%. In still other embodiments, the carbon nanotubes have less than 10% metal oxide impurities.

The carbon nanotubes may be present in the tube composition from about 0.05 wt % to 2 wt % in some embodiments, from about 0.1 wt % to 1 wt % in other embodiments, or from about 0.2 wt % to 0.5 wt % in yet other embodiments. In some embodiments, the tube composition have from about 0.2 wt % to 0.4 wt %, or from about 0.25 wt % to 0.35 wt %.

As discussed above, the tube composition may optionally include other carbon materials such as graphite, carbon black, carbon fiber, and/or milled carbon fiber as another conductor in addition to the carbon nanotubes. In one embodiment, the conductive carbon material is milled carbon fiber. The length of the carbon fibers may vary widely. In some embodiments, the average length of the milled carbon fibers is no more than about 0.5 mm. In other embodiments, the average length of the milled carbon fibers is no more than 0.2 mm. The milled carbon fibers are present in the tube composition from 0 wt % to 20 wt % in some embodiments, from about 1 wt % to 10 wt % in other embodiments, or from about 4 wt % to 12 wt % in yet other embodiments.

The tube composition may contain a variety of other materials that may impact a variety of properties. For example, in some embodiments, the tube composition includes a reinforcement material such as glass fiber to provide rigidity and strength to parts or devices prepared from the tube composition. In some embodiments, the glass fiber is present in the tube composition from 0 wt % to 60 wt %, from about 5 wt % to 60 wt % in other embodiments, or from about 15 wt % to 40 wt % in yet other embodiments.

The tube compositions provided herein are amenable to injection molding processes. After compounding, the tube composition extrudate exits the extruder as a spaghetti-like material that is cooled and cut into small pieces for ease of handling, quick packaging and/or convenient shipping. Alternatively, bulk material could be extruded and used in a bulk process. An injection molding apparatus may then be charged with the extrudate and molded into parts or devices that may be used as is, or require additional machining to refine the device into a useable part. For example, tubing, connectors, threaded connectors, etc. may be prepared by injection molding processes.

Methods of preparing the tube composition are also provided. Methods may include blending all ingredients that will form the tube composition in a hopper and extruding the mixture. Alternatively, the methods may include several steps with the various ingredients being added at any given point in the process. For example, a first amount of a thermoplastic polymer may be compounded with carbon nanotubes in an extruder to produce a first extrudate. The first extrudate may then be blended with glass fiber, carbon fiber, and/or milled carbon fiber and subjected to a second extrusion to produce a second extrudate that is a tube composition. Compounding may take place in a single screw, twin screw, or other style extruder. The methods may also include tumbling all the ingredients of any particular stage of the preparation, prior to being fed to the extruder.

In preparing the tube composition, the same thermoplastic polymer may be used in the preparation of both the first and second extrudates, or different thermoplastic polymers may be used. If different thermoplastic polymers are used in the steps, they may be chemically distinct polymers, or just different grades of the same polymer having varying viscosities, melt indices, or other polymeric properties.

With reference back to FIGS. 1A and 1B, the tube 110 can be manufactured using a variety of manufacturing processes. For example, in a specific embodiment, the tube 110 is injection molded to a desired diameter and length. Alternative manufacturing processes that can be used to make the tube 110 include extruding and roto-molding (also referred to as compression molding).

When the dielectric isolator 100 is used in an aircraft fuel system, it needs to satisfy certain dielectric and lightning strike performance requirements. With respect to dielectric performance requirements, in one embodiment, the dielectric isolator 100 exhibits an electrical resistance of from about 10⁵Ω to 10⁸Ω when measured from end to end at an applied potential of 500 volts DC or greater, with one test probe located on the outside surface of one coupling (e.g., coupling 120 a) and another test probe located on the other coupling (e.g., coupling 120 b). Of course, when used in aircraft fuel systems, the dielectric isolator 100 is expected to exhibit this resistance range from initial installation to the aircraft end of life. Moreover, it will be appreciated that the electrical resistance range may be different depending on the desired application.

With respect to lightning strike performance requirements, in one specific embodiment, the dielectric isolator 100 should not exhibit electrical arcing or sparking, either internally or externally, or voltage/current waveform collapse during and after exposure to multiple (e.g., between ten and thirty) voltage pulses, in either (+/−) polarity, having peak amplitude of 9,000 volts that conform to voltage waveform B in accordance with SAE ARP5412. This simulated lightning strike testing is performed per ARP5416 where one test probe is located on the outside surface of one coupling (e.g., coupling 120 a) and another test probe is located on the other coupling (e.g., coupling 120 b). It will be appreciated that the lightning strike performance requirements may be different depending on the desired application.

To verify electrical robustness, the electrical resistance of the dielectric isolator 100 is measured after exposure to each and every simulated lightning test to verify that the electrical resistance remains within the desired range.

FIG. 2 illustrates a top plan view of another embodiment of dielectric isolator 200. The dielectric isolator 200 is substantially similar to the inline-type dielectric isolator 100 described above and illustrated in FIG. 1 (structure and material), with the exception that it includes a flange 210 attached to the tube 110 between the couplings 120 a,b. The dielectric isolator 200 can be characterized as a bulkhead-type dielectric isolator because the flange 210 permits the dielectric isolator 200 to be attached to a bulkhead 300 of a vessel, such as an aircraft fuel tank, as shown in FIG. 3. To facilitate attachment to the wall 300, the flange 210 includes multiple holes 220 for receiving bolts or other types of fasteners.

The flange 210 can be constructed of a metallic material, such as aluminum or stainless steel. However, it will be appreciated that the flange material is not necessarily limited thereto, provided that the dielectric isolator 100 satisfies the mechanical and electrical requirements of a given installation.

In the illustrated embodiment, the flange 210 is threadingly attached to the ends of the tube 110 via stub ACME threads. Alternatively, the flange 210 can be adhesively bonded to the tube 110. Although many different types of adhesives may be used, one suitable adhesive that can be used to bond the flange 210 to the tube 110 is Araldite AV119 distributed by Huntsman Advanced Materials. It will be appreciated that the flange 210 can be attached to the tube 110 utilizing a variety of other attachment means. For example, the flange 210 can be riveted onto the tube 110 for attachment thereto. In all cases, a protective sealant (e.g., MIL-S-81733 or MIL-S-4383) may be applied to the faying surfaces between the flange 210 and the tube 110 to seal the adhesive bond (if present) from outside elements and to protect the flange 210 from galvanic effects (depending on the material from which the tube 110 is constructed).

When the dielectric isolator 200 is used in an aircraft fuel system, it needs to satisfy certain dielectric and lightning strike performance requirements. With respect to dielectric performance requirements, in one specific embodiment, the dielectric isolator 200 exhibits an electrical resistance of from about 10⁵Ω to 10⁸Ω when measured from end to end at an applied potential of 500 volts DC or greater, with one test probe located on the outside surface of one coupling (e.g., coupling 120 a) and another test probe located on the flange 210. Of course, when used in aircraft fuel systems, the dielectric isolator 200 is expected to exhibit this resistance range from initial installation to the airplane end of life. Moreover, it will be appreciated that the electrical resistance range may be different depending on the desired application.

With respect to lightning strike performance requirements, in one specific embodiment, the dielectric isolator 200 should not exhibit electrical arcing or sparking, either internally or externally, or voltage/current waveform collapse during and after exposure to multiple (e.g., between ten and thirty) voltage pulses, in either (+/−) polarity, having peak amplitude of 9,000 volts that conform to voltage waveform B in accordance with SAE ARP5412. This testing is be performed per ARP5416 where one test probe is located on the outside surface of one coupling (e.g., coupling 120 a) and another test probe is located on the flange 210. It will be appreciated that the lightning strike performance requirements may be different depending on the desired application.

The present invention, thus generally described, will be understood more readily by reference to the following examples, which are provided by way of illustration and are not intended to be limiting.

EXAMPLES

Materials. CF is an abbreviation for carbon fiber, and MCF is an abbreviation for milled carbon fiber, prepared by milling the CF. The CF used in the following tables is Tenax 303 available from Toho Tenax, Rockwood, Tenn. GF is an abbreviation for glass fiber. The GF used in the following is Vetrotex EC 10, available from Owens Corning, Toledo, Ohio. PEEK is an abbreviation for polyetheretherketone. The PEEK used in the following tables is Victrex 150 P, available from Victrex USA, West Conshohocken, Pa. CNT is an abbreviation for carbon nanotubes. The CNT used in the following tables was Nanocyl 7000, available from Nanocyl, Inc.

Each of the examples is a ladder study to determine the impact of the individual components of the compositions. Extruded compositions were prepared by compounding the PEEK with the CNT in a twin screw extruder to form a master batch of 10 wt % CNT in PEEK (MB CNT). The master batch was then further compounded with additional PEEK, and also the CF and the GF in a single-screw extruder. After extrusion and cooling, the compounded materials, were cut to appropriate dimensions.

Comparative Example 1 MCF and GF Constant

Sample PEEK (%) MB CNT (%) MCF (%) GF (%) 1 43 2.0 12 25 2 38 2.5 12 25 3 33 3.0 12 25 4 28 3.5 12 25 5 23 4.0 12 25 6 18 4.5 12 25

Comparative Example 2 CNT and GF Constant

Sample PEEK (%) MB CNT (%) MCF (%) GF (%) 7 60 5 10 25 8 62 5 8 25 9 64 5 6 25 10 66 5 4 25

Example 1 GF Constant

Sample PEEK (%) MB CNT (%) MCF (%) GF (%) 11 67 5 3 25 12 68 5 2 25 13 65.5 3.5 6 25 14 67.5 3.5 4 25 15 68.5 3.5 3 25 16 69.5 3.5 2 25

Example 2 GF, MCF and PEEK Constant

Sample PEEK (%) MB CNT (%) MCF (%) GF (%) 26 67.3 3.7 4 25 27 67.5 3.5 4 25 28 67.7 3.3 4 25 29 67.9 3.1 4 25 30 68.1 2.9 4 25 31 68.3 2.7 4 25

Example 3 Volume Resistivities

Vol. Resistivity Vol. Resistivity at Vol. Resistivity at at 500 VDC 1000 VDC after 1 5000 VDC after 1 Sample (Ω-cm) cycle (Ω-cm) cycles (Ω-cm) 11 1.923E+06  5.123E+07  TC 12 NR/I 1.775E+06  TC 13 NR/I NR/I 5.00E+08 14 NR/I NR/I 7.58E+08 15 NR/I NR/I NR/I 16 NR/I NR/I NR/I 26 2.79E+05 1.76E+05 2.92E+04 27 1.69E+06 8.98E+05 1.07E+05 28 7.81E+08 5.21E+08 2.73E+07 29 — — — 30 — — — 31 — — — TC = Too Conductive to measure NR/I = No Reading as the material was too insulative to determine.

In the above examples, comparative examples 1 and 2 were found to be too conductive for application in a high strike voltage application. As noted, it is desired that upon application of a voltage such as 5,000 volts DC, the composition that is nominally insulative, will exhibit a bulk volume resistivity from about 10³ Ω-cm to 10¹⁰ Ω-cm, which is then a dissipative or shielding material. The other examples are illustrative of the at least some of the breadth of the compositions that meet the desired properties. Other compositions will be evident to those of skill in the art based upon the described embodiments.

One exemplary composition includes a thermoplastic organic polymer, a plurality of carbon nanotubes, and a plurality of carbon fibers, where the composition exhibits a bulk volume resistivity from about 10³ Ω-cm to 10¹⁰ Ω-cm at 5,000 volts DC, and the thermoplastic organic polymer includes about 60 wt % to 75 wt % 4-(4-(4-oxyphenoxy)-phenoxy)-benzoyl polyetheretherketone. In such compositions, from about 60 wt % to 75 wt % of the thermoplastic organic polymer may be present, from about 0.2 to 0.5 wt % of the carbon nanotubes may be present, from about 15 to 40 wt % of the glass fiber may be present; and from about 2 to 7 wt % of the carbon fibers may be present. The composition may exhibit a bulk volume resistivity of about 10⁶ Ω-cm to about 10¹⁰ Ω-cm at 5,000 volts DC. In such materials, the composition may exhibit a bulk volume resistivity of at least about 10⁶ Ω-cm at 100 volts DC. In other embodiments, the tube composition may exhibit a bulk volume resistivity of at least about 10⁸ Ω-cm at 100 volts DC, or, alternatively, at least about 10⁹ Ω-cm at 100 volts DC.

Example 4 Electrical Resistance of Dielectric Isolator

In one particular aircraft fuel system application, dielectric isolators were required to exhibit an electrical resistance of from about 10⁵Ω to 10⁸Ω when measured from end to end at an applied potential of 500 volts DC or greater. To meet these requirements, inline-type dielectric isolators, similar to the ones described above and illustrated in FIGS. 1A and 1B, were injection molded from a material that contained 67.5 wt % PEEK, 0.35 wt % carbon nanotubes, 4 wt % carbon fibers, and 25 wt % glass fiber. The dielectric isolators were molded into cylindrical tubes having three different nominal diameters: 0.75 inches, 2 inches, and 4 inches. Aluminum ferrules that conform to SAE AS5836-1-XX (flex) and SAE AS1656-1-XX (rigid) and finished with a chemical conversion coat per MIL-C-5541 were attached to the ends of the tubes via stub ACME threads. The overall length of each dielectric isolator was 5.4 inches from the end of one ferrule to the end of the other ferrule.

Each dielectric isolator was tested for dielectric performance (pre-lightning test) by measuring the electrical resistance of the dielectric isolator from end to end at an applied potential of 500 volts DC or greater (as high as 6,000 volts DC), with one test probe located on the outside surface of one ferrule and another test probe located on the other ferrule. As shown in FIG. 4, the test results show that a very large percentage of the dielectric isolators met the resistance requirements from about 10⁵Ω to 10⁸Ω. However, as indicated by the process variability bandwidth in the graph of FIG. 4, full coverage would not exist for 0.5 inch diameter isolators as well as 4 inch diameter isolators. To meet the electrical resistance requirements across the entire size matrix, it has been proposed to provide two slightly different tube compositions—one tube composition with a slightly higher carbon fiber content (e.g., 4.2 wt %) and another tube composition with a slightly lower carbon fiber content (e.g., 3.8 wt %). Generally, by increasing the carbon fiber content in the composition, the electrical resistance of the isolator decreases.

After completing the measurement of the electrical resistance of the dielectric isolators described above, the isolators were then exposed to simulated lightning events consisting of twelve voltage pulses, six in each polarity (positive and negative), having a peak amplitude of 9,000 volts that conform to voltage waveform B in accordance with SAE ARP5412. The testing was performed per SAE ARP5416 with one test probe located on the outside surface of one ferrule and another test probe located on the other ferrule. To pass the test, each isolator should not exhibit electrical arcing or sparking, either internally or externally, or voltage/current waveform collapse during and after exposure to the twelve voltage pulses. At the conclusion of the testing, all of the isolators passed the simulated lightning strike test as none of them exhibited an arc, spark, or waveform collapse.

Each dielectric isolator was then retested for dielectric performance (post-lightning test) by measuring the electrical resistance of the dielectric isolator from end to end at an applied potential of 500 volts DC or greater (as high as 6,000 volts DC), with one test probe located on the outside surface of one of the ferrules and another test probe located on the other ferrule, to ensure that the electrical resistance for each isolator was still between about 10⁵Ω and 10⁸Ω. As shown in FIG. 5, the test results show that a large percentage of the dielectric isolators met the resistance requirements of between about 10⁵Ω and 10⁸Ω following the simulated lightning strike test.

While several, non-limiting examples have been illustrated and described, it should be understood that changes and modifications can be made therein in accordance with ordinary skill in the art without departing from the invention in its broader aspects as defined in the following claims.

It will be appreciated that the dielectric isolators 100, 200 described above are particularly suited for use in aircraft fuel systems because they allow enough current to flow therethrough to prevent electrostatic charge build up due to fluid flow through or around the outer surface of the fuel line, yet limit the flow of indirect current associated with voltage therethrough during a lightning strike to current levels that are relatively low and safe for the aircraft fuel system. They are electrically robust in that repeated lightning events should not materially affect their electrical resistances. Additionally, because a substantial portion of the tube composition includes PEEK, the dielectric isolators 100, 200 are lightweight, yet have a high mechanical strength. Moreover, since the dielectric isolators 100, 200 are injection molded, one or more of the following benefits can be achieved: quick process cycle time, inexpensive process cost, minimal porosity issues, and reduced scrap.

It will be appreciated that the dielectric isolators 100, 200 described above are not only useful in aircraft fuel systems, but also can be utilized in many other applications including, without limitation, petroleum and other fluid distribution applications.

For the purposes of this disclosure and unless otherwise specified, “a” or “an” means “one or more.” To the extent that the term “includes” or “including” is used in the specification or the claims, it is intended to be inclusive in a manner similar to the term “comprising” as that term is interpreted when employed as a transitional word in a claim. Furthermore, to the extent that the term “or” is employed (e.g., A or B) it is intended to mean “A or B or both.” When the applicants intend to indicate “only A or B but not both” then the term “only A or B but not both” will be employed. Thus, use of the term “or” herein is the inclusive, and not the exclusive use. See, Bryan A. Garner, A Dictionary of Modern Legal Usage 624 (2d. Ed. 1995). Also, to the extent that the terms “in” or “into” are used in the specification or the claims, it is intended to additionally mean “on” or “onto.” Furthermore, to the extent the term “connect” is used in the specification or claims, it is intended to mean not only “directly connected to,” but also “indirectly connected to” such as connected through another component or multiple components.

As used herein, “about” will be understood by persons of ordinary skill in the art and will vary to some extent depending upon the context in which it is used. If there are uses of the term which are not clear to persons of ordinary skill in the art, given the context in which it is used, “about” will mean up to plus or minus 10% of the particular term. From about X to Y is intended to mean from about X to about Y, where X and Y are the specified values.

While the present application illustrates various embodiments, and while these embodiments have been described in some detail, it is not the intention of the applicant to restrict or in any way limit the scope of the claimed invention to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention, in its broader aspects, is not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicant's claimed invention. Moreover, the foregoing embodiments are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application. 

What is claimed is:
 1. A dielectric isolator comprising: a tube constructed of a composition including from about 50 wt % to 98 wt % of PEEK, from about 0.05 wt % to 2.0 wt % of carbon nanotubes, and from about 1 wt % to 10 wt % of carbon fibers; a pair of couplings attached to opposing ends of the tube; and wherein the dielectric isolator exhibits a bulk volume resistivity from about 10³ Ω-cm to at an applied potential of 5,000 volts DC.
 2. The dielectric isolator of claim 1, wherein the tube composition further comprises glass fiber.
 3. The dielectric isolator of claim 2, wherein the tube composition includes from about 5 wt % to 60 wt % of the glass fiber.
 4. The dielectric isolator of claim 2, wherein the tube composition includes from about 60 to 75 wt % of PEEK, from about 0.2 to 0.4 wt % of the carbon nanotubes, from about 2 to 7 wt % of the carbon fibers, and from about 15 to 40 wt % glass fiber.
 5. The dielectric isolator of claim 1, further comprising a flange attached to the tube between the end couplings and configured to be attached to a bulkhead.
 6. The dielectric isolator of claim 5, wherein the flange is constructed of aluminum and attached to the tube via stub ACME threads.
 7. A dielectric isolator comprising: a tube constructed of a composition including about 50 wt % to 98 wt % of PEEK, about 0.05 wt % to 2.0 wt % of carbon nanotubes, 1 wt % to 10 wt % of milled carbon fibers, and glass fiber; a pair of couplings attached to opposing ends of the tube; a flange attached to the tube between the pair of couplings and configured to be attached to a bulkhead; wherein the dielectric isolator exhibits a bulk volume resistivity from about 10³ Ω-cm to 10¹⁰ Ω-cm at an applied potential of 5,000 volts DC; wherein the dielectric isolator exhibits electrical resistance from about 10⁵Ω-10⁸Ω when measured at opposite ends of said tube; and wherein the tube exhibits a five-strike bulk volume resistivity degradation of no more than a factor of 50% at an applied potential of greater than 5,000 volts DC. 